Steel wire rod for bearings having excellent drawability and coil formability after drawing
View Patent ↗A steel wire rod includes, in terms of mass %, 0.95-1.10% C, 0.10-0.70% Si, 0.20-1.20% Mn, 0.90-1.60% Cr, 0-0.25% Mo, 0-25 ppm B, 0-0.020% P, 0-0.020% S, 0-0.0010% O, 0-0.030% N, and 0.010-0.100% Al. In a surface area of the steel wire rod, the Vickers hardness is HV 300 to HV 420, the area ratio of pearlite is 80% or more, and the area ratio of pro-eutectoid cementite is 2.0% or less. In an inner area of the steel wire rod, the area ratio of pearlite is 90% or more, and the area ratio of pro-eutectoid cementite is 5.0% or less. In the steel wire rod, the area ratio of pearlite blocks having an equivalent circle diameter of more than 40 μm is 0.62% or less, and the difference in Vickers hardness between the surface area and a center portion is HV 20.0 or less.
1. A steel wire rod comprising:
C: 0.95 to 1.10 mass %,
Si: 0.10 to 0.70 mass %,
Mn: 0.20 to 1.20 mass %,
Cr: 0.90 to 1.60 mass %,
Mo: 0 to 0.25 mass %,
B: 0 to 25 ppm,
P: 0 to 0.020 mass %,
S: 0 to 0.020 mass %,
O: 0 to 0.0010 mass %,
N: 0 to 0.030 mass %,
Al: 0.010 to 0.100 mass %, and
a balance: Fe and impurities,
wherein a surface area is an area between a surface and a line 0.1 times a half of an equivalent circle diameter of the steel wire rod apart from the surface in a cross-section perpendicular to a longitudinal direction, and has a microstructure consisting of pearlite, pro-eutectoid cementite, and a balance,
wherein in the surface area, a Vickers hardness is HV 300 to 420, an area ratio of the pearlite is 80% or more, an area ratio of the pro-eutectoid cementite is 2.0% or less, and the balance is one or more selected from the group consisting of ferrite, spheroidal cementite, and bainite,
wherein an inner area is an area enclosed by the line 0.1 times the half of the equivalent circle diameter of the steel wire rod apart from the surface and including a center in the cross-section perpendicular to the longitudinal direction, and has a microstructure consisting of pearlite, pro-eutectoid cementite, and a balance,
wherein in the inner area, an area ratio of the pearlite is 90% or more, an area ratio of the pro-eutectoid cementite is 5.0% or less, the balance is one or more selected from the group consisting of ferrite, spheroidal cementite, and bainite, and an area ratio of pearlite blocks existing in the pearlite and having an equivalent circle diameter of more than 40 μm is 0.62% or less, and
wherein a center portion is an area enclosed by a line 0.5 times the half of the equivalent circle diameter of the steel wire rod apart from the center and including the center in the cross-section perpendicular to the longitudinal direction, and a difference between a Vickers hardness of the center portion and a Vickers hardness of the surface area is HV 20.0 or less.
2. The steel wire rod according to claim 1 , further comprising at least one selected from the group consisting of:
Mo: 0.05 to 0.25 mass %, and
B: 1 to 25 ppm.
3. The steel wire rod according to claim 1 , wherein a diameter of the steel wire rod is 3.5 mm to 5.5 mm.
4. The steel wire rod according to claim 2 , wherein a rod diameter of the steel wire rod is 3.5 mm to 5.5 mm.